St Albans South has just acquired a combined CD/RA indicator, and an OFF indicator, both Westinghouse fibre-optic type. On powering them up, I was a little surprised to find them showing yellow indications rather than white. A look at the Railsigns website say that yellow was used for 'rearward facing' indicators. Does this mean they were ones that faced back towards the driver? Only I thought the CD/RA one was always facing the train driver anyway!

I'd be grateful for some information about their use. The OFF indicator - was this always lit when the appropriate signal was showing a clear aspect, or only when the platform TC was occupied by a train?Likewise, presumably the CD and RA indicators could only be lit when the OFF indicator was lit. Could they only be operated in the sequence CD > RA, please?

I've never met CD indicators, so can only speak for the OFF and RA indicators.

You're quite correct, the OFF indicator may only be illuminated when the signal to which it applies is showing proceed on the main aspect; subsiduary aspects are not included AFAIK. They can also work with a mechanical signal, which also must be showing OFF. TC occupation is not normally included - it's usually a simple repeat of the signal - though there are odd instances of TC occupation being part of the control, for example at Smethwick Galton Bridge Low Level.

The RA indication illuminates when the RA plunger on the platform is depressed (signal must be off), and usually remains lit until the signal is replaced. The lighting of the RA indicator extinguishes the OFF indicator (never the two lit together).

I would expect the sequence involving CD indicators to be: 1) Train Ready to Start plunger operated by station staff, 2) signaller receives request and clears signal, 3) OFF indicator illuminated so station staff can continue with despatch, 4) CD plunger operated by station staff, 5) CD indicator lit and so driver closes doors, 6) station staff confirm safe to despatch and press RA plunger, 7) OFF extinguishes (and I assume CD does too) RA lights up and train departs.

The platform starting signals at St Pancras had rear facing indicators when re-signalled to West Hampstead PSB (and maybe prior to this as well, others will confirm or deny this I am sure).

This was because a loco coming onto the back of the inbound service would sometimes end up with the cab (or even the whole loco) beyond the signal legitimately as the signals where not right at the end of the platforms, so the driver would not be able to see the "face" of the signal when awaiting departure. Hence the "rear facing" indicators.

Sharpey wrote: " The platform starting signals at St Pancras had rear facing indicators when re-signalled to West Hampstead PSB (and maybe prior to this as well, others will confirm or deny this I am sure). .... "

Correct me if wrong, but these sound like perpetuation of the pre-West Hampstead resignalling starters of St. Pancras box, which replicated the route indications displayed at the front of the signal, and were indeed, as opposed to the tendency of some older 'theatre' indicators to appear to be 'off-white' rather than 'lunar white' (no brilliant white LEDs then of course) deliberately yellow-ish (officially 'orange'?). And IIRC, they also featured at some earlier power installations, like York and Newcastle, but I don't know if there are/have been any at-signal rear-facing CD or RA indications; though I suppose there could have been a need for rear-facing OFFs.

Although some of the following details may have been altered in the latter days of changes to traditional roles of 'who does / is responsible for, what', it is my understanding that OFF indicators, though perhaps most frequently found on or associated with, station platforms, can be necessary elsewhere, e.g. for shunter/guard/driver information in some sidings / yards where methods of movements and/or the siting or application of a siding(s) exit signal(s) may be difficult to see or is(are) unclear.

But where an OFF indicator was provided in connection with train despatch from a platform, it was normally intended for guard and/or platform staff use : [ I believe that if drivers needed a repeat of a platform starting signal, it ought always to have been a Banner Repeater signal, remembering that, at through platforms, the repeating may have also been a necessity for non-stopping trains.]

Although I wouldn't dispute most of Mark's comments, my own experience of these does not agree with his account in all respects.

In at least some places, with the early introduction of DOO working of passenger trains, the initial 'Right away' indication was merely an illuminated 'R' [ the irretrievable 'R' as one safety-worried ops. manager disparagingly called it, because there were no CDs then, and illumination of the 'R' allowed the train's driver to close doors and the train then to leave providing only that his/her interlock light lit in the cab, and there was no way to cancel the 'R' if the staff saw anything unsafe on closure of the doors (it was only extinguished by the signal going to red), which is still the case with RAs now, but perhaps this is appropriate, as once having permission to depart drivers ought to then concentrate on signals and the line ahead, and not on whether they should watch to see if the RA is extinguished ]. Only in later days did the standard indication become RA (actually not the first time this had been used to indicate 'Right away': e.g. until the mid-1960s, platforms at Paddington had "RA" indications in association with the still-working ex-GWR colour-light signalling). And introduction of the CD then meant that the RA could solely mean 'safe to depart'. In more recent years, doubtless as a safety measure, the RA may instead have to be operated by platform staff using their special key to turn, and then release, a spring-loaded key switch (the same would apply to the CDs where provided : Even the control of the OFF often became protected if still a plunger (e.g., in a locked cupboard), or was also by key switch : (Pre-1989 resignalling of Liverpool Street, it had not been unknown for experienced, observant commuters on a train whose departure was being delayed, to nip out and press the 'Train ready' plunger, presumably hoping this would hasten the start of their journey home!)

Mark Lamb wrote: " .... The RA indication illuminates when the RA plunger on the platform is depressed (signal must be off), and usually remains lit until the signal is replaced. The lighting of the RA indicator extinguishes the OFF indicator (never the two lit together). .... "

In my experience, OFFs always remain lit until the associated signal goes to Danger. My impression is that if CD and or RA facilities are present as well as an OFF, that the OFF would stay lit while the signal is Off : I feel pretty sure that I have seen, somewhere, instances of OFF and RA lit simultaneously. I would have thought that extinguishing the OFF when an RA illuminated could cause concern to some staff that the signal had gone back to Danger, but I cannot be totally certain on this.A footnote on modern OFFs : Where these are provided on a bi-directional platform line, it is not uncommon, in order to avoid confusion, to see that the display is OFFDN or OFFUP, indicating which direction the OFF is lit for : This is obviously particularly necessary if there is a starting signal at both ends of the platform, and/or the OFF indicators are themselves double-sided and simultaneously show the same indication(s) 'fore and aft'.

Mark Lamb wrote: " .... I've never met CD indicators, so can only speak for the OFF and RA indicators. .... "

As touched on above, CD indicators were introduced (indication now often displayable by the same indicator as the RA; there never being a really valid reason for CD and RA to be visible at the same time) to enable the RA to become a post-doors closure, simpler, positive indication to drivers that it was safe for the train to depart.As to when CDs are operated, I'm unsure what current standard practice is for new installations regarding whether they can be lit with the platform signal still at Danger. In these days of extensive assessment of risks, the signal may have to be Off in case a driver somehow misread CD and took it to mean 'right away' (i.e. signal Off), but I feel sure that there are (have been?) some busy locations where, to optimise platform occupancies, CD could be operated to get doors closed before the signal cleared: The perhaps most obvious such situation would be 'a.m. peak' at busy termini where arriving passenger trains depart as empty coaching stock. A 'free' CD would then, while waiting for the departure signal to clear, allow platform staff, once sure that everyone had alighted, to use the CD to have the driver close/lock the doors ready for a prompt departure on clearance of the signal, and thus saving precious seconds in getting the platform cleared for another arrival.

So I'd suggest that Mark's sequence be modified as follows (my alterations in red) : - " I would expect the sequence involving CD indicators to be: 1) Train Ready to Start plunger or key switch operated by station staff, 2) signaller receives request and clears signal when safe and correct to do so, and OFF indicator illuminates so station staff can continue with despatch, 3) CD plunger or key switch operated by station staff when safe to close doors, 4) CD indicator lit allows driver to close doors, 5) if CD operated by key switch, station staff release switch when doors closed, CD extinguishes, and station staff confirm safe to despatch and press RA plunger or key switch, 6) CD (if plunger-operated?) extinguishes, RA lights up and if driver's in-cab interlock indication is illuminated (= all doors properly closed), train can depart."

Thanks for the various replies. I'm thinking about incorporating them into our 1970s CLS demo. The latter needs to be recontrolled along the lines of our recent LED installation to overcome minor problems which have developed over the 5 years the demo has been running, so it seems an opportune moment to include these indicators.

But much of the operation described above seems to be applicable to a terminal station, particularly the use of a "Train Ready" plunger to tell the signalman the train is ready and to clear the platform signal. At a through station the signal would, I assume, already be showing a clear aspect and therefore the 'OFF' indicator would already be lit. (My query about TC control of this indicator is because I note they use 55w car headlamp bulbs - which seems to pose the potential problem of getting rather hot internally if on for long periods! We may well substitute a 5w LED reflector lamp.)

TRTS plungers are likely anywhere that trains are likely to spend a significant amount of time at a platform, I'd have said. Certainly they were provided in the 1951 installation at York (and presumably in the present setup too), I think I've noticed them at New Street, I think Peterborough may have them too and possibly Cambridge, Simsig Exeter and Westbury have them so (given Geoff's insistence on accuracy) I think we can take it that's so in real life, and so on.

Sharpey wrote: " The platform starting signals at St Pancras had rear facing indicators when re-signalled to West Hampstead PSB (and maybe prior to this as well, others will confirm or deny this I am sure). .... "

Correct me if wrong, but these sound like perpetuation of the pre-West Hampstead resignalling starters of St. Pancras box, which replicated the route indications displayed at the front of the signal, and were indeed, as opposed to the tendency of some older 'theatre' indicators to appear to be 'off-white' rather than 'lunar white' (no brilliant white LEDs then of course) deliberately yellow-ish (officially 'orange'?). And IIRC, they also featured at some earlier power installations, like York and Newcastle, but I don't know if there are/have been any at-signal rear-facing CD or RA indications; though I suppose there could have been a need for rear-facing OFFs.

The 'stand off indicators' at St pancras also existed under the old St Pancras PSB and were indeed yellow in colour using a yellow outer glass rather than the usual lunar white glass.

CD indicators (which are operated by a key or plunger adjacent to the TRTS key or plunger) do not require the platform starting signal to be OFF to operate. this is so that the platform staff can get the doors closed at any time (like for instance in the case of empty stock where there is a strong desire not to get passengers on a train bound for the carriage washer!).

John Webb wrote:St Albans South has just acquired a combined CD/RA indicator, and an OFF indicator, both Westinghouse fibre-optic type. On powering them up, I was a little surprised to find them showing yellow indications rather than white. A look at the Railsigns website say that yellow was used for 'rearward facing' indicators. Does this mean they were ones that faced back towards the driver? Only I thought the CD/RA one was always facing the train driver anyway!

I'd be grateful for some information about their use. The OFF indicator - was this always lit when the appropriate signal was showing a clear aspect, or only when the platform TC was occupied by a train?Likewise, presumably the CD and RA indicators could only be lit when the OFF indicator was lit. Could they only be operated in the sequence CD > RA, please?

John Webb wrote: " .... But much of the operation described above seems to be applicable to a terminal station, particularly the use of a "Train Ready" plunger to tell the signalman the train is ready and to clear the platform signal. At a through station the signal would, I assume, already be showing a clear aspect and therefore the 'OFF' indicator would already be lit. (My query about TC control of this indicator is because I note they use 55w car headlamp bulbs - which seems to pose the potential problem of getting rather hot internally if on for long periods! We may well substitute a 5w LED reflector lamp.) "

Apologies if my reply was overlong or wandered too far away from your original questions John.

To perhaps more directly address your queries, OFFs, and RAs (or its equivalent), have been around for a considerable time, whereas I believe I'm correct in saying that CDs appeared only during the last approx. thirty years in connection with refining Driver Only Operation.As to the Railsigns describing the 'yellow'(?) indications as being for 'rearward-facing', I know that 'rear-facing' [i.e. on the rear of signals - (you could say facing forward) - for Drivers of locos standing beyond signals to look back at]route indicators were purposely a different colour [ (I thought 'orange', or perhaps "amber"); to avoid confusing drivers of trains coming the other way]; I am less sure that OFF and other indicators were provided in a similar way.

I am certain that there have been OFF indicators lit for long periods, as some applied variously to Automatic signals or Controlled signals with Auto working facility for 'the straight road', though these were of one or other 'Stencil' types, and I don't know what sort of lamps and voltages/wattages were involved.

The normal CD/RA sequence would indeed be to display CD first, with RA then second, though I've no idea if the equipment enforced this or it was just down to station staff discipline : I have encountered Fibre-optic and/or LED CD/RA indicators which show the two indications through the same area of the same aperture/matrix, so the CD has to disappear once acted upon, before the RA can be put up, and I thought that this sequence was the norm irrespective of the hardware/s in use (but I cannot say with confidence what happens where there are, or used to be, instances of the CD and RA being separate display units where the CD had been added after an RA was already in use).

It is perhaps worth mentioning that, depending on platform length/alignment / signal position and methods of train crewing and working, there will be examples of OFF indicators where there is no CD/RA, and v.v. (CD/RAs with no OFF), and even still possibly RA with no CD.

The point about TRTS (or TRS : Tran Ready To Start) : True, almost inevitably found at staffed and busy termini platforms, but can also be found at many major stations' through platforms, and others where it could also be critical not to set route for a train to leave if it was not ready.One prime example known to me was (I think it's gone now, as circumstances have changed) was the Down platform at Hitchin, which was on the Slow line, where one of the platform starting signal's routes was for calling passenger trains to cross all the ECML tracks to reach the Letchworth/Royston/Cambridge line; this was also a common crew-relief point, and for a time some of the trains also carried station-to-station parcels : - Down and Up Inter-City trains coming to a dead stand because a train signalled across to the Down Cambridge line did not leave for several minutes after 'time' was distinctly unpopular ! [ Although provision of the TRTS was not necessarily 100% successfully effective; if operator discipline was such that someone frequently pressed the TRTS plunger as the train came to a stand before realising that there were two or three wheelchaired passengers to alight or the relief crew hadn't arrived yet ! ]

Last edited by StevieG on Fri Sep 9, 2016 11:40 am, edited 10 times in total.

John Webb wrote:But much of the operation described above seems to be applicable to a terminal station, particularly the use of a "Train Ready" plunger to tell the signalman the train is ready and to clear the platform signal. At a through station the signal would, I assume, already be showing a clear aspect and therefore the 'OFF' indicator would already be lit.

TRTS plungers were employed at the previous signalled layouts Paddington,Liverpool Street and Kings Cross where a buzzer sounded in the box and a green light was lit at the end of the platform line on the diagram

Back facing indications are amber, not yellow (a classic interview question is "where is the colour amber used in railway signalling?"). Contrary to an earlier post an RA indication supplements and OFF indication - it will not cause an illuminated OFF indication to extinguish.